(2) Aircraft count: Only available aircraft are
assigned to the network.
(3) Balance: Each aircraft type is assigned to the
same number of flight legs arriving at a station as
departing that station.
Each possible assignment of an aircraft type to a flight
leg is represented by a binary variable, with value 1
if the aircraft type is assigned to that flight leg, and
0 otherwise. The resulting formulation for the fleet
assignment problem of a major airline contains about
20,000 rows and 30,000 columns and can be solved
typically within minutes.